Image sensors are widely used in a variety of consumer electronic devices, including digital cameras and cellular telephones with built-in digital cameras. An image sensor may comprise a matrix of sensor elements. Image quality may be degraded if one or more image sensor elements are occluded.
An occlusion may result from a dust particle becoming lodged against an image sensor element or on a cover glass in front of the image sensor. The occlusion may block some or all light that might otherwise be incident to the image sensor element. If all light is blocked, the image sensor element output signal may remain fixed as the level of light incident to the image sensor element changes. If the incident light is only partially blocked, the image sensor element output signal may vary as the incident light varies. In the latter case, however, the output signal may not be as large as if the image sensor element were not partially blocked.
“Brightness of light” may be referred to hereinafter using the more precise term “illuminance.” Units of lux, or lumens per square meter, may be used as a measure of illuminance. An image sensor may convert incident illuminance to analog image sensor element output signals. An analog to digital converter (ADC) may convert the image sensor element output signals to a digital format. A least significant bit (LSB) associated with the ADC may establish a granularity with which an illuminance level sensed by an image sensor element may be measured. Thus, in the field of digital imaging, illuminance may also be expressed in LSB units.
Some commercially available methods may operate to identify unusable image sensor elements at a production facility following fabrication. For example, each production image sensor may be tested and calibrated under controlled conditions. Such methods may incur substantial cost, and may fail to identify occlusions resulting from dust particles that accumulate in an imaging system after production.
Consider, for example, a cellular telephone with a built-in camera. The camera image sensor may have been tested and calibrated at a semiconductor manufacturing facility following fabrication and before shipment to a cellular telephone manufacturer. During the process of incorporating a camera module assembly including the tested image sensor into the cellular telephone, a dust particle inside the camera module may break free and land on the surface of one or more image sensor elements. The resulting occlusion may subsequently manifest itself as a cluster of dark pixels on captured images. Thus, a need to identify and compensate for such occlusions may exist.